A standard overload clutch such as described in my U.S. Pat. No. 4,467,902 based on German patent 3,009,244 filed 11 March 1980 has a driven and a driving clutch member both rotatable about a clutch axis. The members have adjacent endless tracks that together form an annular groove centered on the axis and each member is formed with a plurality of seats that open into the track groove. The seats of the members are identically angularly spaced so that each can be aligned with a respective seat of the other member to form therewith a pocket open into the groove. Rollers, typically balls or wheels usually carried on a cage can either roll around the tracks or sit in the seats. A spring braced against the cage or a pusher ring urges these rollers into the pockets in which position they rotationally link the two members.
At least one side of each pocket is inclined, that is nonradial of or nonparallel to the clutch axis, so that when torque is being transmitted from the driving member to the driven member a component of this force urges the balls out of the pockets against the spring force. When the torque transmitted through the clutch exceeds a predetermined limit this component exceeds the spring force and the balls move out of the pockets and into in the track where they are confined in lodgements, thereby decoupling the two members. Such a clutch is useful, for example, in a piece of agricultural equipment where a feed must stop when jammed with crop so that the machine operator can clear the jam. To reseat the rollers in the pockets it is necessary to reverse rotate one of the members, thereby moving the recesses back into alignment and allowing the spring to push the rollers back into the thus reformed pockets. In some arrangements such reverse-rotation does not reengage the clutch, but other means are used.
It is also possible to set the clutch up to lock when overloaded. In such an arrangement the balls, after being cammed out of their pockets by a torque overload, can move angularly limitedly between the two members before moving against an abutment that locks the two members relative to each other, even though the torque being transmitted substantially exceeds the overload-cutout force. In this type of arrangement some sensor, for instance a rotation-detecting switch, is employed to shut off the drive during the short time between when the clutch disengages and when it locks up. This sensor can also work with a controller to open up an upstream clutch or transmission, or to otherwise stop rotating the driving clutch member.
My earlier patent describes a system where a cage is used having separate lodgements for the rollers. This cage is not only a fairly expensive item to manufacture, but must be positioned centrally in a location which ideally should be used to transmit torque. The pusher element that exerts the spring force on the rollers is formed like the race of an axial-force roller bearing so that if rollers are between the track of this element and the clutch member track it takes very little force to put the rollers back into the pockets. Vibration or some other minor external shock can, therefore, close up the clutch.
Another arrangement described in German patent 32 08 182 uses a separate cage for the rollers. The clutch members are constructed so that there is little axial room left in the device. As a result it is necessary to make the recesses very shallow and the flanks relatively flat, which type of construction either greatly limits the upper limit of the overload torque or requires that an extremely powerful spring be used. In addition the rotational mounting of the clutch members requires structure taking up quite a bit of room and even so there is substantial rolling friction between rollers, the pusher element pushing them in one direction, and the clutch members they roll on. As a result of the space constraints there is virtually only point contact between the rollers and the pusher element.
My earlier patent further suggests forming one of the clutch members with two angularly limited tracks for each pocket to receive the rollers when the clutch is overloaded.
As a result of the angular extent of these formations, such construction limits the amount of rollers that can be used, thereby of course limiting the torque that the clutch can transmit while increasing the amount of wear the rollers are subject to.
It is also possible as described in above-cited German 3,208,182 to limit the angular movement of the cage relative to one of the clutch members by providing a pin projecting either from the cage or the one clutch member and engaging in an angularly extending slot of the one clutch member or cage. This allows one to shorten the path of the rollers so that more such rollers can be used. Such a system allows only very small roller balls to be used so that since the force-transmitting potential of balls used as rollers is equal to the square of their diameter such an arrangement only has limited load potential. In addition such an arrangement is not usable at high speeds as it is offcenter and would be torn loose if it were used to stop the considerable momentum of a rapidly rotating cage and balls.
There is another system in my above-cited patent used to reduce torque after overload cutoff described which operates in either rotational sense. This arrangement has the problem that, if reversed to reengage the clutch, it is possible to move the rollers past the pockets if one moves too rapidly, that is the rollers move right over the pockets from their lodgements to one angular side of them to their lodgements on the opposite angular side. Thus it is frequently necessary to gently reverse the drive many times before the clutch reengages, a procedure that obviously puts considerable wear on drive components while doing no useful work. Such an arrangement is virtually unusable with an automatic piece of equipment.
When such a clutch is used in a system where the two members are only to be coupled in one or more predetermined angular positions, the overall clutch rating is substantially limited. Furthermore when the device is used in an arrangement where the clutch goes through an alternating torque after being overloaded, it is necessary to provide expensive planetary gearing that takes up quite a bit of room.